The present application relates to a formulation and method useful, e.g., in forming a passivation layer as occurs in semiconductor manufacturing; more particularly, to a formulation and method that facilitates more complete removal of crosslinker, as commonly used in the formation of a passivation layer, under less aggressive processing conditions.
In Far Back End of Line (FBEOL) portion of a semiconductor device processing sequence, a passivation layer is often employed for protective purposes by providing a stress, water, and electrical barrier to the underlying wiring. Among other things, the passivation layer plays a role in preventing chip-package-interconnection (CPI) failure. Commonly, a photosensitive polyimide (PSPI) is used to form the passivation layer because of its thermal and chemical stability, and mechanical strength. The formation of the passivation layer normally entails a multistep process.
First, the PSPI is typically formed by spin-coating a combination of a base polymer—usually an esterified polyamide resin with photosensitive groups, e.g. methacrylate functional groups, linked to the carboxylic group—and a crosslinker, which is often a methacrylate, such as hydroxyethyl methacrylate (HEMA), to form a film on the pertinent surface of the semiconductor device at FBEOL. This, and other aspects typically entailed in this processing now discussed are illustrated in FIG. 1. Film formation is usually followed by exposure of the film thus formed to ultraviolet (UV) radiation. The UV exposure causes the crosslinker, e.g. HEMA, to polymerize with the photosensitive group linkage on the base polymer, e.g., the esterified polyamide resin, forming the crosslinking network. The crosslinking enables photo-imaging, and the result is a photosensitive polymer: those areas that are crosslinked become insoluble to developer, whereas those areas that are not crosslinked will be washed away by developer.
Then, after photo-imaging and developer wash, a thermal cure process is usually performed whereby the esterified polyamide converts to a polyimide structure breaking the ester bonds and losing linkage to the crosslinker polymer (hereinafter known as “Polymer 1”), e.g. poly(HEMA). Polymer 1 may include both the homopolymer of the crosslinker and the copolymer of the crosslinker and the photosensitive linkage on the esterified polyamide resin. This conversion, also known as imidization, is typically performed at temperatures of about 350° C. to about 385° C., usually for a period of about 4 hours for the former temperature, to about 1 hour for the latter.
Also during the thermal cure, Polymer 1, e.g. poly(HEMA), de-polymerizes, and the monomer that ensues volatilizes and is baked out of the passivation layer in an effort to remove it. However, not all of the crosslinker, including its monomer, can be easily removed from the cured PSPI. Such monomer that remains can further evaporate, polymerize and subsequently contaminate product wafers and the manufacturing hardware, such as process chambers, used to generate them. Moreover, while aggressive processing conditions, such as a high temperature PSPI cure, e.g. in the range of about 385° C., can more effectively remove residual crosslinker, an attending consequence is a strong degradation in device performance, especially in pFET devices.
In view of the above, there is a continuing need for providing more complete removal of crosslinker from PSPI passivation layers using less aggressive processing conditions, including a lower cure temperature.